1. Field of the Invention
This invention relates to an improvement in the preparation of polymers of high impact strength and enhanced processability. More particularly the invention relates to an improvement in the bulk polymerization of vinyl halide or vinyl halide-comonomer mixtures in the presence of high concentrations of high molecular weight polyolefins. It is especially concerned with a novel improvement in said polymerization which diminishes formation of product agglomerates and provides a more finely divided, more homogeneous, and more easily processed particulate polymer.
2. Description of the Prior Art
It is known to polymerize a vinyl halide, e.g. vinyl chloride, or up to about 50 weight percent of mixture thereof with a compatible comonomer in bulk in the presence of a polyolefin to obtain a vinyl halide polymer of improved impact strength and processibility. Thus, according to copending U.S. application Ser. No. 746,046 filed Nov. 30, 1976, of A. Takahashi which is a continuation-in-part of copending U.S. application Ser. No. 674,202, filed Apr. 5, 1976 now U.S. Pat. No. 4,071,582, which, in turn, is a continuation-in-part of U.S. application Ser. No. 427,895, now abandoned, filed Dec. 26, 1973, as a continuation-in-part of Ser. No. 251,099 filed May 8, 1972, now abandoned, vinyl halide or a vinyl halide-comonomer mixture can be polymerized in a single or two stage bulk reaction in the presence of about 0.05 to about 20% of a polyolefin rubber of weight molecular weight ranging from about 50,000 to 1,000,000 or higher to produce a polyvinyl halide product containing both free, i.e. dispersed, and grafted polyolefin of excellent impact strength, and other desirable properties such as reduced melt viscosity. It is now found, however, that polymers of especially excellent high impact strengths of the order of about 10 to 20 foot-pounds per inch or greater, are obtained as a general result in the reference process when the amount of polyolefin charged is above about 3.5 weight percent, preferably above about 5.3 weight percent based on the vinyl halide concentration. However, it is also found that use of such high concentrations of polyolefin, especially of polyolefins of molecular weight above 150,000 incurs difficulties in mixing the reaction mass, and results in a product having a relatively low proportion of finely divided particles and a substantial number of relatively massive lumps or agglomerates. In the reference polymerization, as soon as the agitated reaction mixture is warmed up to initiate polymerization, the polymerization mass proceeds from a substantially clear solution or dispersion of polyolefin in vinyl halide or vinyl halide-comonomer mixture to a milky, opaque emulsion. After about one hour, the reaction becomes a paste, and after about 1.5 hours of reaction, corresponding to about 25 to 30% conversion of vinyl halide to polymer, the reaction isotherm shows a rapid increase, with concurrent thickening of the paste, i.e. development of a "thick paste state" in the reaction mass. After about 40 to 45% conversion of the vinyl halide monomer to polymer, the thick paste becomes, in the main, a fine non-viscous powder. When high concentrations of polyolefin are charged to the reaction in order to obtain the aforementioned polymer of exceptionably good impact strength, the thick paste state of the reaction mixture is so viscous, i.e. of consistency substantially similar to unbaked dough, that the agitation of the mixture provides little mixing effect in the polymerization mass. In other words, the reaction mixture consists of several large dough-like agglomerates, or in extreme cases, a single dough-like lump, adhering to, and revolving or rotating upon the agitator or stirrer. When the reaction is carried to completion from the aforementioned thick paste state, the product contains a relatively small proportion of evenly shaped finely divided particles compared to the proportion of such fines obtained when the polyolefin is charged at low concentrations, i.e. at 5.3% by weight or less or especially at 3.5% by weight or less based on the vinyl halide employed. The product also contains, one or several massive irregularly shaped agglomerates which agglomerate or agglomerates can in extreme cases comprise a major or a predominant part of the polymer product. The particulate product of the reference process normally has hard fused surfaces and generally large particles therein must be comminuted, e.g. by grinding or equivalent pulverization process, to make them suitable for conventional polymer processing steps. The latter operations generally entail handling the polymer in a fluid melted state. Accordingly, polymer processing of the aforementioned large particles or massive agglomerates entails undesirable, costly expenditure of mechanical energy in pulverization of the particles. Alternative direct melting or fusion of the products containing the massive agglomerates and a large proportion of relatively large particles also generally requires prolonged heating of the product which can affect deleteriously the polymer color and/or degrade the polymer. The large product particles and agglomerates obtained by employing the aforementioned relatively high amounts of polyolefin in the reference process are generally less homogeneous than small product particles since high local concentrations of monomer, polyolefin and reaction initiator build up within the large particles and especially within the massive agglomerates absent effective mixing within these large particles during their formation in the thick paste state. Additionally, poor heat transfer from within the relative massive body of these particles deleteriously affects the homogeneity of the polymer within the particle.
The reference process generally prescribes agitation of the polymerization mass, but as pointed out above and as illustrated in Example 2 below such agitation does not prevent formation of product agglomerates and undesirably large proportions of large particles in the product when the aforementioned large concentrations of polyolefin are employed in the polymerization. Moreover increasing the speed of the agitation in the reaction would not be a feasible method of overcoming the aforementioned disadvantages of the reference process, since in the thick paste state of the polymerization wherein the undesirable agglomerates and large particles arise, the dough-like reaction mass collects upon, and revolves with, the rotating or revolving agitator or stirrer. Accordingly, increasing the speed of the agitation would not increase the internal mixing in the agglomerated reaction mass and might, especially at extremely high agitation speeds, damage the agitator or the agitator motor.